Ask HN: iOS engineers, what are your most helpful coding practices

iOS engineers, we all know that iOS code can get pretty messy, crashy, buggy and almost impossible to debug. What are some of the coding practices that you used that have helped your app to more well written? I will consolidate your suggestions into an github page. Some of things that I can think of are 1) Switch statement should always be exhaustive. 2) Avoid state as much as possible 3) Singleton's are evil

Love to learn from your experience!